Method and means for detecting cloudiness of transparent material



y 31, 1951 J. c. FROMMER 2,562,131

METHOD AND MEANS FOR DETECTING CLOUDINESS OF TRANSPARENT MATERIAL Filed Nov. 5, 1948 4 Sheets-Sheet l DEV/CE M24 AMPLIFIER g2: INVENTOR.

JOSEPH EON/YER BY F/g 9 Fig fig July 31, 1951 C. FROMMER J. 2,562,181 METHOD AND MEANS FOR DETECTING CLOUDINESS OF TRANSPARENT MATERIAL Filed Nov. 5, 1948 4 Sheets-Sheet 2 L6 INVENTOR. 2/0 L A 204 ./05EPH kONf/EK July 31, 1951 J. c. FROMMER 2,562,131

METHOD AND MEANS FOR DETECTING CLOUDINESS OF TRANSPARENT MATERIAL 4 Sheets-Sheet 5 Filed Nov. 5, 1948 INVENTOR. /015 EON/75R July 31, 1951 J. c. FROMMER METHOD AND MEANS FOR DETECTING CLOUDINESS OF TRANSPARENT MATERIAL 4 Sheets-Sheet 4 Filed Nov. 5, 1948 and INVENTOR. Jasnw C. Fkomvm Fatenteci July 31 1 951 UNITED STATES PATENT OFFICE "METHOD AND MEANS FOR DETECTING CLOUDINESS F TRANSPARENT MA- TERIAL Joseph C. Frommer, Cincinnati, Ohio Application November 5, 1948, Serial No.- 58,423

defined. If the. medium contains, suspended pare 'izicles, the sharp contour lines will be blurred. in :the :following description '2. medium which passeszblurred images .will be referred to as a cloudy medium and by more or less cloudiness there will be understood more or less blurringof ;the .image .formed under given conditions across acertainthickness of such medium. Conversely, byiclarity of a medium there is meant its capability of transmitting clear images and by more orbless clarity is meant sharper or less sharp xiefinition ofzimages formed across such medium.

This invention has for one of its primary objectstthe detection of suspendedforeign materials in liquids or gases indicated by the cloudiness caused by such suspensions but it lends itself also to .the detection of other phenomena, whether physical or chemical, causing blurring of images transmitted through the medium to be investigated. The information'obtainedfrom such detection can be used to actuate some signal, alarm, automatic correcting or other device; togive quan- :titative information on the amount of cloudiness; to actuate some automatic device in responseto changes ofthe amount'of-cloudiness; or for some other similar purpose.

'Cloudiness of transparentmedia is ordinarily accompanied .byareduction of the total amount oflight transmitted across a certain thickness of the material under investigation. Thisreduction of light transmission is sometimes great enough to be measured by ordinary photoelectric means; forcertain uses, however, especially in thechemical industry, it would be necessary to detect cloudiness also when such cloudiness is accompanied 'by a variation in light transmission far Lbelow that which would be caused by random variations of the light absorption of the mate- ;rial under investigation. It is an object of the invention to provide a method and means for detecting also such cloudiness aswill cause only a negligible decrease of light transmission.

Another object of the invention is to provide -A further object .of the invention is to provide aphotoelectric amplifier that gives a signal depending on the modulation of light reaching the photoelectric cell but substantially independ- .ent otthe averageamountof illumination. 4

LA further-object of the invention is to provide .anoptical system-suitable.for oneor more of the purposes outlinedabovewhich does .not require readjustment of :the;apparatus if :materials "haviing different indices .of refraction are tested.

Another object of .the invention .is to provide the opticalsystem withreflecting means which allows the placement of at least .allthose optical parts,the-relative position of which toward each other is critical, on one side of the medium tobe tested.

Another object of the inventionis to provide a system that gives-a quantitative indication of the cloudiness of themediumunder test. 1

Another object of the invention is to provide a system 'thatwill initiate aneaction dependingin magnitude onthe quantitative amount of cloudii- =ness of the medium under test. r i

Another object of the invention is to provide optical targets suitable for the purposes outlined above. I

Still another object of-the invention is to'meas ure cloudiness by focusing light on sharply defined areas-having :at-leastone dimension small compared to the distance'ofthese .areasfrom the objective lens (or mirror) thatfocuses thelight on them, an'd'to detect the-light scattered due to cloudiness on nearby'areas'on'whichno light was focused. Several embodiments of the invention are illustrated by way of example in the accompanying drawingsjnwhich: v Fig. '1 shows inelevation, partly in section; an .optical system according .to one form of vthe in' vention; n I

Figs; 2,3, 4, 5,.a'nd 6 represent various'formscf optical targets; Figs. '7 and 8 show jtwo modified p i Fig. .Qrepresents time curves of the light out.-

' put obtainable froman incandescent lamp fed by -a device sensitive to variations of cloudiness but not sensitive, to variations of light absorption of transparent media.

. :A furtherobject of the invention is to cause =-the modulation of light falling on a photoelecsignal; 7

alternating current;

Fig. 10 represents time curves of "light obtain:- able by choppingthe output of a constantsource of light; j i

Fig. 11 represent time curves of a photoelectric ,Fig. 12.1 a plan view and'Fig. 13 is an eleva;

tric cell to change substantially with the clouditional .view o'f-another modified optical system;"

nessof the material under test.

Figs. 14, 1'5, and l6show, respectively, in'a'n elevation, a side view and a plan view a modified optical target;

Fig. 17 shows an amplifier circuit;

Fig. 18 illustrates a modification of the circuit shown in Fig. 17;

Fig. 19 illustrates in a fragmentary longitudinal section a structural embodiment of a device according to the invention; and 19Fig. 20 is a section along the line 2020 in Fig.

Referring first to Fig. l, I denotes a lamp having a filament 2, 3 is a condenser lens, 4 a first optical target, 5 a first objective lens, and 6 a container carrying the medium 8 to be investigated. The container 6 has two plan-parallel, transparent walls I and l, 5' denotes a second objective lens, 4' a second target, 3 a second condenser lens, 9 a photoelectric cell surrounded by a shield Ifl having a hole l2, II a motor adapted to move the target 4 when coupled therewith, H an adjusting device, and i3 an amplifier. The first optical target 4 is situated in the focal plane of the first objective lens 5 and the second target 4 in the focal plane of the second objective lens 5.

The term target as used in this description denotes an optical element having transparent and dark portions, the term transparent denoting in this connection an area or area of the target capable of directing light comin from a light source towards a photoelectric cell, and the term dark denoting an area or areas of the same target capable of preventing said light from said light source to travel toward said photoelectric cell.

When the target 4 is illuminated by the lamp l, its image is thrown into infinity by the objective lens 5. The rays carrying this image traverse the medium 8 and are refocused by the objective lens 5' into the plane of the second target 4. The targets 4 and 4 may comprise alternating transparent and dark areas in the form of parallel clear Stripes 4| and opaque stripes 42 as shown in Fig. 2. The patterns of the targets 4 and 4' are so laid out that, assuming the motor II to be at rest in a certain position, the images of all transparent portions of 4 fall on dark portions of 4'. Therefore, as long as the medium 8 is entirely clear (assuming ideal optical conditions throughout the system) no light will reach the photoelectric cell 9. If, however, the medium 8 becomes cloudy, the light rays traversing it are deflected from their original course and some of the rays focused on the dark portions of the second target 4' will reach instead the transparent portions thereof. Thus, a certain amount of light reaches the photoelectric cell 9, and the amount of this light is a measure of the cloudiness of the medium 8.

It will be noted that by holding the motor II at a standstill in another position it would also be possible to focus the transparent portions of 4 on the transparent portions of 4, in which case cloudiness of the medium 8 would cause a decrease of the light falling on the cell 9, but in general it is preferable to focus the transparent portions of 4 on the dark portions of 4 because the change caused by a certain amount of variation of light is more readily detected if the change occurs from entire or almost entire darkness than if it occurs from a high level of illumination.

The photoeletric current obtained in 9 can be amplified by any known method; its value can be indicated by a meter; it can be recorded; it can actuate correcting means or signalling means; or it may serve any other desired function.

The motor H is of such a nature a to move the target 4, when coupled with the latter, in the plane of this target in such a way that the transparent portions of 4 are focused alternatingly on transparent and dark portions of 4. With a target as illustrated in Fig. 2, a mechanical vibrator is used as the motor l I. The height of the vibrations may most advantageously be equal to one full pitch of the target pattern so that during each stroke there will be alternation from full coincidence of the image of the transparent portion of 4 with the transparent portions of 4 to full coincidence of the image of said transparent portions of 4 with the dark portions of 4.

As long as the medium 8 is clear there will be theoretically zero illumination of the phototube 9 during the coverage of transparent with dark and maximum (full) illumination during the coverage of transparent with transparent, and the photoelectric signal generated by the phototube will be as represented schematically by the curve ice or the curve Ibo of Fig. 11. If the medium 8 becomes cloudy then some of the rays focused toward the dark portions in the dark period will be diverted to transparent portions and some of the rays focused toward the transparent portions during the light period will be diverted toward dark portions. Accordingly, the maximum of i1.- lumination decreases and the minimum thereof increases as compared with the conditions which exist when the medium 8 is clear and the signal at the phototube assumes the shapes shown schematically at Zac and Zbo in Fig. 11. If the cloudiness is so heavy that the image formed at 4' is entirely blurred, then the light reaching the phototube will stay constant during the entire cycle of vibration and the photoelectric signal will be as shown in Fig. 11 at 3110 and Sbo (horizontal lines).

One example of an application of the invention be described now: Light is produced by one filamerit of a type 2330 Mazda Lamp arranged in front of a plane-convex lens 3 and is thrown by this lens onto a diameter first target 4 at a distance of 1 from said lamp filament. The target 4 has alternating light and dark stripes of .0075 width which may have been obtained for instance by the reproduction of a magnified drawing of the pattern on photographic film. The target is held in the focal plane of a color corrected lens 5 of 32 mm. diameter and 122' mm. focal length. The second target 4 and the second objective lens 5 are respectively identical with 4 and 5. The container 6 of 10" in length has a clear diameter of 1 and contains the media to be tested, which in the experiment referred to was in one case oleic acid and in the other case mineral oil. For certain chemical purposes these chemicals must be of a very high purity and the presence of for instance /25 of 1% of suspended activated bleached earth, called Filtrol, would make either of these chemicals unsuitable for their purposes even though the addition of so small an amount of Filtrol causes only so slight a reduction of the light transmission that with ordinary photoelectric means I was unable to measure this reduction in transmission. As it is well known, commercial oleic acid is a dark red or yellow liquid, whereas mineral oil is colorless. With this arrangement:

(a) the average light obtained on the phototube across oleic acid is materially different (less) from the average light obtained on the phototube across mineral oil;

(19) the relative modulation obtained from air,

asearei aol ic acid. and clea mineral oil .is .in all ree cases the same. By relative modulation ere-,understooda number; proportional to the I between;peak toreakyariationof:the photoectricflgnal and the average =va-lue'of the siganal;v and to.) the relativermodulation obtained with oleic acid Qrmineral oilrcontaining lessthan 25 of 1 .ofiF-iltrolfalls ,well below half Y of :the. relative modiula on obtained with the clear chemica if .it is .desired that the :apparatus should not reunite readjustment whenever a substanceof uiii erentabsorntionis to be testedfithenan ampliiiershouldbe used-which is responsive to relative .Iznodulation-only. An amplifier-of this kind will pedescribed .later with reference to Fig. .17.

" .The amplification of alternating voltages is easier than the amplification of direct voltages and hotoelectric amplifiersareoitendesigned ,to take advantage of the modulation present in .the .l ebto tput .o inca descent lamps .fed by alter- ,nat ng current. .Fig. 9 illustrates schematically what may be called the natural modulation of light -.emitted by an incandescent lamp fed by alternatingcurrent. With lamps fed at the line ,iregujenoy of .60 cycles thenatural modulation is only in the .crder of 10%. Therefore, to obia nili h which substantially higher moduflated. mechanical .choppersare used quite often thatalternatinglyinterrupt the light from a con- .stan i hts urce. Such nventional c p rs would. however, not be suitable for producing the result required in the I present invention because, they would afford no discrimination between'the variations of the photoelec gnal caused by variations i i ht absorption of the medium li n th one hand an v ri tion cau ed by cloudiness on the other hand, the relative modulation remaining constant with such conventional method of modulating the signal. In contrast thereto the method according to the present invention, which .makes relative-modulation dependent on the cloudiness of the medium to be tested, gives the desired discrimination above mentionedabsorption affecting in this case only the average photoelectric current, and cloudiness affecting only the relative modulation. Another"importantfeature of the invention reside's in'that the two targets are arranged in the ifgcalplanesof the respective objective lenses and therefore the loci of the images of the targets in the space between the two objective lenses are in infinity; This feature allows the use of the same equipment for the testing of materials having *widely difierentindices of refraction without the -need-of different adjustment of the optical system for such difierent media. Generally speakchanges in the index of refraction of the space through which the rays travel will cause a change of the focusing distance, the difference being the larger the smaller the focusing distance itself is. The error decreases with increasing focusing distance and it becomes zero if the image is focused into infinity as is the case in the present invention where the targets are situated in the focal planes of their coordinated objectives. While the condensing lenses 3 and .3 are not indispensable, the lens 3 improves the efficiency .=of the illumination .of .the target .4 and the lens 3' allows to .place the phototube 9 at .a more nonvenient spot and to better protect it .against outsidelight. Bestresults are obtained from the first condenser lens 3 if causes the image of {the filament :2 to appear midway between and rc. "The position of such ima e will necessarily @a different index of refraction .and therefore for :best results, .this image should be. focused to that midpoint between 5 and-5 :foranzaverage. index of refraction. I I I I .The oscillatory movement of :the target-4 .can be replacediby a rotary-movementif patterns are vused on the targets :4and A that have :ciroular symmetry. A portiomof suchapatternisshown in Fig. 3. Itcontains radial segments which are .alternatingly transparent 4| and dark 42. :It can be rotated around its .axis of symmetry -43 bythe motor H; I

.The widthof the stripesof thepattern shown .in Fig. ,3 varies in :the radial direction. Formu- :rnerical definition ofscloudinessit is an advantage .-to refer to blurring of stripes .of uniform :width. Centrally symmetrical stripes of uniform width can beeobtained bylevolvents asshownschematically in Fig.4. I

Light can be caused to reach ornot to. reach the photo cell .9 not only by the use :of transparent and dark areas on the targets 4 and .41. but also by forming on them areas that divert the light into different directions by refiectionor refraction. .A targetof such natureis shown in Fig. .5 in front view and .in Fig. 6 .in elevation. This target is made of "a ,piece of transparent material, preferably glass, whose one side fl, :is flat, .and whose other side has parallel furrows. The sides 46, 41 of these furrows "with the com.- mon fiat side 48 form small prisms thatdivert parallel rays toward one direction or :the other, according to the inclination of the furrow walls. Fig. '7 illustrates ,an application of such targets as shown in Figs. 5 and ,6. This embodiment of the invention comprises two lam s, la. and lb, having filaments 2a and 2b, two first-condenser lenses 3a and 3b, a first target 4 a first objective lens 5, asecond objective lens 5', aisecond target 4', .two second condenser lenses 3a. and 3b, and two phototubes 9aand 9b. The targets 4 -and4' are placed in the focal'pla'nes of Eand .5, respectively, and so adjusted that the images .of all furrow walls 46 of 4 slanting one way are focused on furrow walls 46 of 4' and th images of all furrow walls 41 slanting the other way are focused on the furrow wallsA'l' of .4 The file,- .ments 2a and 2b are so arranged thattheobjective 5 is reached by light emanating fromfilament 2a across furrow walls 46, and by rays emanating from 2b across furrow Walls 41, The photoelectric cells 9a and 9b are so positioned that 9a is in the path of those rays of the light from 5 which reach the furrow walls 46' and are diverted thereby whereas .9?) is in the path of those rays which reach the furrow walls 4'! and are diverted by these walls. Consequently, as long as the medium between 5 and 5 is clear, the phototube 9a is illuminated only .bylight from 2a and the phototube 9b is illuminated only by light from 2b. 2a. and 2b are connected to the alternating voltage source 2|, .22 across rectifiers 23, 24 connected in opposite polarity so that zone filament draws current only during one half period of the alternating voltage cycle, and the other filament draws current only during the other half period. Corresponding to the modu- J'lation of these sources of light the signal on the two phototubes will be modulated in opposite phase. If, however, the medium between the objectives 5 :and .5 is cloudy, then part of .the light focused originally on one set .of furrow walls will be dispersed toward the other set and the two Opposite modulations will tend to compensyary when themedium 8 is changed to one haying sate each other causing diminution of the relative modulation of both phototubes. If the absorption of the medium between and 5' changes, the light falling on both phototubes will change proportionally, but the relativ modulation will remain unaltered. Again an amplifier, generally indicated at I3, may perform the amplification necessary for a desired indication or other action in function of said modulation.

The maintenance of the exact relative position of the targets 4 and 4 to each other and to the lenses 5 and 5' is critical in the embodiment shown in Fig. 7, because a slight chang in this relative position may cause a material portion of the light originally focused on one set of furrow walls of 4 to fall on the other set. In the embodiment illustrated in Fig. 8 the light of the filament 2 which falls on one set of the furrow 'walls of the target 4 is directed by the objective lens 5 across the medium to be investigated (not shown in this figure) and is then reflected by two mirrors [4 and I4 across the same medium back to objective 5. The two mirrors l4 and I4 are positioned at exactly 90 to each other and have the line of intersection of their planes parallel to the furrows on 4. According to the position of 4 with respect to the optical axis of 5 light rays emitted by the filament 2 which are directed by the furrow walls 4'1 toward the objective 5 are refocused either entirely, partially or not at all on furrow walls 48 of 4, and these furrow walls 46 direct the light coming from the objective lens 5 toward the phototube 9. The target 4 serves in this case also the function for which in the previously described embodiment the second target 4' is provided. The motor II, which may be any mechanical oscillating device, causes th target '4 to vibrate in its own plane and thereby to alternatingly increase and decrease the illumination 'of 9. Upon the medium between 5 and mirrors I4 and I4 becoming cloudy, the sharpness of focusing and the modulation of the photoelectric signal will decrease. This system of Fig. 8 has the advantage of not being sensitive to slight displacements of the mirrors i4 and I4 relative to the rest of the optical system, so long as these mirrors remain in exact relation to each other: in the plane of the drawing, the movement of one mirror is compensated by th equal movement of the other, and in the plane perpendicular to the plane of the drawing the exact alignment is not critical, as slight misalignment would merely focus rays destined for a certain point of a furrow wall to another point of the same furrow wall. A single mirror may be used instead of the two mirrors I4 and I l if this single mirror is mechanically mounted so that displacements of the mirror relative to the other parts of the optical systems will not occur.

The two individualadvantages of the two embodiments just described with reference to Figs. 7 and 8, i. e. absence of moving parts in the case of Fig. 7 and insensitivity to slight displacements of the parts on one side of the medium to be investigated relative to the parts on the other side of said medium in the case of Fig. 8, are combined in the optical system represented in Figs. 12 and 13. In these figures the reference numerals la and lb again denote lamps, 9a and 9b phototubes, 5 an objective lens, and 4 a, target, whereas 3a, 3b, 3a and 3b denote condenser lenses and I4 denotes a prism whose back sides I44 and I45 are silvered. The medium 8 to be tested is kept in the chamber 6 having plan parallel windows :1 and I held in place by a flange and by a.- ca 54. The cap 64 also protects the prism I4. The'target 4 has on one side par allel furrows with walls 46 and 41 and on its other side two prisms 48, 48 which divert the passage of rays upwardly and downwardly. The diversion by the prisms 48, 48" is effected in a plane which is perpendicular to the plane in which the furrow walls 46, 41 divert the light. Whilst the prisms 48, 48' divert light from both lamps Ia and lb across the upper half of the optics, and the light reflected to the lower half of the optics down toward the two phototubes 9a and 9b, the furrow walls 46 and 4'! direct the light from one lamp toward one phototube and from the other lamp toward the other phototube. The light of la and lb is modulated in opposite phase, e. g. by having the filaments 2a and 2b fed from an AC voltage source across rectifiers in opposite polarities. Consequently, as in the embodiment illustrated in Fig. 7, the two phototubes are illuminated by light modulated in opposite phase as long as the medium 8 is clear, and this modulation decreases if the medium gets cloudy.

The function of the condenser lenses 3a; 3b, 3a and 31) may be performed by the target 4 if the angles of the furrow walls 46, 4? are so staggered that the small prisms formed by them will concentrate parallel rays into the desired focus, and the two prisms 48, 48 on the back side are given a curvature to perform such focusing in the other plane. Figs. l4, l5 and 16 show schematically a target 5 shaped in this manner.

The previously described optical systems of Figs. 1 and 8; if their targets are oscillated or rotated, provide an alternating photoelectric signal the modulation of which is the higher the clearer the medium 8 under testis, and a conventional amplifier may be used in connection with these optical systems. However, if an indication is wanted which is indicative of cloudiness alone by being independent of the absorption of the medium, then a circuit has to be used which is responsive to the relative modulation of the photoelectric signal only, i. e. to the proportion between the alternating and the average photoelectric current. Fig. l7 shows an amplifier answering this requirement of being responsive to the relative modulation only. This amplifier contains photoelectric cell 9 having a cathode lfill and an anode I'll; a first amplifier tube I02 having a cathode Hi3, a first grid I04, one or more additional grids of which One-is shown at W5, and an anode IE5; a second amplifier tube I82 having a cathode Illl, a grid ill, and an anode H2; a diode IIG having a cathode Ill and an anode I ill; a last amplifier tube I2l having a cathode I22, a first grid I23, one or more additional grids of which one is shown at I24, and an anode I25; and a voltage stabilizer tube I21 having a cathode I23 and an anode I29, The heaters of the amplifier tubes are fed from a conventional source of heater current, not shown, which may be held constant by some conventional voltage or current regulator, and a conventional supply of plate voltage, not shown, provides plate voltage, whose negative terminal is indicated at M! and whose positive terminal is indicated at hi2. Thecathode I03 of .tube 162 is connected to the negative terminal I 01' the B-supply. and its anode is connected across a load resistor Ilil tothe plate I29 of the voltage regulator I2'I. The anode Illl of the phototube 9 is connected directly to the plate I29 of I21, whereas the cathode Hill of the phototube .9 is connected to grid I64 of tube I02. The

screen grid I of: tube I 02 is connected across a resistor I44 to the plate I29 of the voltage re plator tube I 2.1- and across the condenser I63 to the negative terminal I4I of the B-supply. The second tube I02 has connected its cathode -I I'll across a resistor I I3 to the negative terminal I of the B-suppl-y. its grid III' across a condenser I45 and a resistor I09 to line MI and its plate II2 acrossresis-torss I I-4 and M4 to theplate I129" of the voltage regulator I2-"I. The cathode H21 of the diode I I-S is connected to the nega tive. terminal I41 of: the. B-supply. and its anode I I8 is connected. toa tapon' a bank of resistors I159: extending between line IM and a condenser N5, the other terminal of the latterbeing connected' to a tap ona: resistor H4. The grid I23 of. c the last amplifier tube I=2I is connected throughraresistor I to the anode I I8 of the diode H6andacrossacondenser I3I to I M. The cathode I22- of tube I-2I- is connected to I M and its'se'cond grid I24 to the plate I29 of the stabilizer tube I27, whereas its anode I is connected across a relay I83 to the positive terminal I42 of theB-s-upply; The relay I83 hasa normally closed.- contacti I32, which is connected between- 'arvoltage source- I34 and a; signal device I33. It will be noted that the cathode I00 of the cell 9 is connected to no otheripoint than: to the grid I204- of the first tube I02 and that therefore the phdtoelectri'c current passes between grid I04 and cathode IIl3' of this-tube, allowing that-grid to become positive with". respect to its potential forizero current by as'much as corresponds to the amount of photoelectric current passing; The relation between grid voltage: and grid. currenti's expressedl'by'the equation:

inwhich I stands for the grid current, a for the base of the natural logarithms and V for the voltage difference between grid' and cathode and iii which I0 is a constant" (dimension current) and Vi) and U are other constants (dimension voltage), all depending on the tube characteris tics" and the temperature of thecathode. The above expression'can be written:

Thus, if. the grid current varies between the vialuesIg-i and I r, the voltage will vary between the limits:

V1=Vo+U 16gIb1-Ul0g I uand: the: variation in voltagewillbe "It. will be seen. that the variation: in voltage obtained. on the grid I04 depends only ou the prc ortionbetweenl Igl and: Ig2-' and not on the absolute values of these: currents; The aboveformula would yield infinite negative for 1 :0. Actually, creeping currents prevent the grid from becoming more negative than about 1 volt underany' pradtic'al conditions. That limi tau-cu: would still destroy "theindependence fromaverage level" of illuminatic m were: it not that even with; the utmost care: the illumination a't theoretically zero will be in the order of 20% of the average illumination; Should the optics He perfectedso as to give too low minimumlevelsg'the pattern could alwaysbe designed so that a high enough level of illumination is main-' 't'a iiie at' 'the mihii-num points.

The rest of the circuit of Fig. 17 is conventional. The alternating voltage generated on the plate I06 is further amplified in the trio'de I02, and detected in the diode H6. The output of that diode is filtered by the resistor I20 and condenser I3 I and brought to the grid of the third amplifier tube I2I. According to the points at which the resistor bank IIS and the resistor II4 (serving for line adjustment) are tapped oil, it will take a certain amount of relative modulation to generate a sufiiciently high negative voltage on grid I23 of the third amplifier tube I2I to cut off its current and cause the relay I83 to drop out and close contact I32. Condensers I45 and I64 serve to suppress unwanted alternating voltages, e. g. those that are caused by the natural modulation of the lamp I if the modulation of focusing is done at a frequency substantially lower than C. P. S.

The amplifier circuit of Fig. 17 may also be used in connection with the optical systems illustrated in Figs. 7, 1-2 and 13, by connectingeither phototube 9a or 92) of said optical systems in place of phototube 9 of Fig. 1'7. The other phototube could then beomitted. But better results, especially the elimination of errors caused by a difierence in light-emission of the two lamps Ia and Ib, are obtainableby amplifying the signals of both phototubesBa and-9b. An amplifier doing this and at the same time takingadvantage of the fact that inthe systems of Figs. '7 and 12 and 13' the modulation of light occurs at line frequencyis illustrated in Fig-.- 18. In this circuit, theanodes I'IllJa' and I00b of the phototubes 9c and 9b are connected to the plate I-29' ofthe voltage stabilizer tube I21 and their cathodes IOIa and Hill) to the grid "Ma and I-Il4b' of the tubes IllZa and: I'02b. The cathodes I03d and 131) of the tubes Ill2a and I 02b are connected to the negative'terminal I M of the-B supply and their screen grids H1511 and [05b across a common resistor I44 to the anode I29 of the voltage stabilizer'tube I2'I. The plates of the tubes I'DZccand I021 are connected across load resistors Ill-Ia and Hill) to the' anode I29 of the voltage stabilizer tube I21 and through con-- densers I08! and I 08!) to the anodes II8a and H817 of a double diode H6. The cathodes Til and IIlb" of this double" diode I IB' are connected acro'ss condensers I 52a and I527) to a line I64 which isheld at an alternating voltage withrespect tothe negative end I4'I' of the B-supply; said alternating voltage being derived from" a; powertransformer I 59" (whose conventional other windings are not shown) by a vo1t'age divider I56, I51. All four electrodes of said twin diode II 6 are also connected across resistors IISa; I I91), I 3'5a. and I35b to a point I'lIl which is-held ata positivevoltage with} respect to the negative end I4I- of the B-sup'pl y by a voltage" divider con-- sisting of resistors I6I and I152; The cathodes I I 1a. and II'Ib of the diode I I6 are connected further acrossresistors I20d and I 206 to grids I23aand" I231) of a' twin amplifier tube' I2-I". The cathodes I220; and I221) of this twin amplifier tube are connected across a common resistor I5I' to the negative terminal of the B-supply and their anodes I2-5aiahd I251) across resistors I530 and I531) to the'positive terminal I32 of it. Biz-- tween the anodes I25a and I 2 5b there is connecteda meter I54. O

this circuit the phototubes are connected directl to the control grids IIl'4 a and I041) of the tubes- I020;- and? I025, and there is no other ath provided-for the photoelectric current from 11' l the cathodes of the phototubes. Therefore, in analogy to what was pointed out in connection with Fig. 17, the alternating signal obtained on the control grids will be a function of the rel ative modulation of the light falling on the phototubes, and hence of the cloudiness of the medium under test. The signal on these grids is amplified on the plates I06a and I061) and brought through condensers I08a and llltb to the plates H80, and liBb of the twin diode IIIi'. Each diode of the twin diode IIIi has between its respective cathode and anode the algebraic sum of the alternating voltages applied to these electrodes. As the alternating voltages impressed on the cathodes are equal, these algebraic sums will be equal to each other if there is no modulation present on the anodes, and they will be altered in opposite directions if modulation of equal and opposite directions to the alternating voltages applied to the cathodes Illa and M121 is impressed upon the anodes Ba and H817. It has been shown in connection with Figs. '7 and 12 and 13 that as long as the medium 8 is clear there take place modulations of line frequency and of opposite polarities of the light falling on the phototubes 9a and 9b. These modulations of light will cause alternating voltages of opposite polarity on the grids I04a and I041? and, consequently, alternating voltages of opposite polarity on the anodes Mia and H161), which, in turn, are transmitted to the anodes IIBa. and II 8b of the double diode IIB. Therefore, the alternating voltage difference between cathode and anode of one diode system will increase and the alternating voltage difference between cathode and anode of the other system will decrease, causing correspondingly higher and lower rectified signals on the cathodes I Ila and I I'll) of the double diode IIE'. These rectified signals are transmitted across the resistors I20a and I2!!!) to the grids I23a and I23b of the twin triode I2 causing a corresponding diiference in the anode currents flowing in the two systems. This difference, which is the measure of clarity (freedom from cloudiness) of the medium under test, can be read on the meter I54.

If the device according to the invention is to be used in a chemical plant to test the cloudiness of a liquid flowing in a pipe system, then installation, maintenance, and servicing require that the container 6 be integrally connected to the piping system and that the optical system form one integral part that can be mounted rigidly in a well defined position relative to said container, but which can still be removed without opening the piping connections to the container 6. It is further desirable that fastening of either the container 6 or of the optical assembly should cause no stress that would upset the alignment of the optical system. Figs. 19 and 20 illustrate an installation which complies with the requirements just mentioned in which the liquid 8, which may be filtered through a filter press not shown in the drawing, is gathered in a trough 20I supplying a pipeline 202. To force a substantial portion of the liquid flow across the chamber 6, a dam '203 is provided in the trough 20 I, and to each side of this dam there is a pipe 62 and 63, respectively, both said pipes 62 and 63 leading to the chamber G. 204 is a flat table, e. g. in the form of a steel channel fastened securely to the floor of the building or to the base of the filter press. The chamber 6 is fastened to said table 204 by U-bolts 205 resting upon the upper half of the chamber whose lower end is supported by cradles 206, which in turn are held in place by tubular spacers 201 provided around the legs of the U-bolts 205.

The optical system containing lenses, light sources, phototubes, amplifiers, etc. is housed in two housings 208 and 208 that are rigidly held in position with respect to each other by a common bottom 20!! and by braces 2 I 0. These braces are fastened to the bottom portion 209 in any suitable manner and to the walls of the two housings 201 and 201 by brackets ZI I. The bottom 209 is provided with four holes 2I2 to clear the legs of the U-bolts '205 and the spacers 20! provided around these legs. The bottom 209 can be fastened to the table 204, such as by screws 2I3. To protect the optical portions from dust, dirt and fluids, two flexible or pliable covers 2I4 and 2M are provided to fit over both the chamber 6 and the tubes 2I5 and 2I5 which house the optical systems. Further protection is provided by split covers ZIB and 2H that can be fastened from both sides over the space between the two housings 208 and 208, and which are cut out to clear the pipe connections 62 and 63.

The chamber 6 may be heated, if the chemical process so requires. To protect the optical and electronic parts from that heat,-the inner walls of the housings 208 and 208 may be lined with heat insulator sheets 2I8 and 2I8'. A pipe 2I9 with a cock 220 is provided to drain the chamber 6, and clearance holes 22I and 222 in the bottom 209 and in the table 204 respectively, allow space for said drain pipe, or the liquid drained out of it.

In most of the embodiments described hereto fore the alternating areas that direct light to one phototube or the other, or transmit or absorb light, were substantially parallel stripes. These areas may have other shapes without departing from the invention; they may, for instance, be in the shape of a checker board. Whatever their shape, however, it is preferable to have at least one dimension of such areas small compared with their distance from the objective which focuses their image. According to the degree of cloudiness to be detected this dimension may be chosen smaller or wider, a very good sensitivity for many practical purposes being achieved with such dimension being in the order of or less of said distance from the objective. The dark and transparent portions need not be of equal width; relative modulation can be increased by making the dark portions slightly wider and average 11- lumination may be increased by making the transparent portions wider. A magnified or reduced image of the first target may be projected on a second target of correspondingly larger or smaller size. Focusing of the image by parabolic mirrors or the like, replacing the incandescent lamp by other light sources, diverting the light by rows of inclined mirrors instead of-by prisms, keeping the target 4 in motion instead of 4, or other changes in the components or their arrangement may be made without departing from the invention.

What I claim is: t

1. A method of detecting cloudiness of a transparent medium, comprising the steps of directing light across the medium to be tested, gathering on a photoelectric cell both a first portion of said light consisting of rays passing through said me-.

. 13 unmodulated, and detecting'the relative'modul'ationoi' the photoelectric current caused by the combined action of said two portions of said light on said photoelectric cell. 1 2. A method of detecting cloudiness of media having transparency, comprising the steps of directing light from a source onto a first target having transparent and dark portions, focusing an image of said first target across the medium to-be tested onto a second target having likewise transparent and dark portions, moving the first target relativel to .the second target to vary the incidence of the images of the transparent and dark portions of said first target upon the transparent anddark portions of said second target, and detecting the relative modulation of the light passing both said targets. 1' 3. A m thod as claimed in claim 2, wherein the first target is moved in its own plane.

4. An apparatusfor' detecting cloudiness of media having transparency, comprising at least one light source, a first target adapted to be illuminated by said light source, a second target, both said targets having transparent and dark portions, optical means including at least one objective for projecting an image of said first target across the medium to be tested onto said second target, means to periodically vary the incidence of illuminated and dark portions of the image formed by said first target with light and dark portions of said second target and photoelectric means adapted to be influenced by light passing said second target and to detect the relative modulation of said light passing said second target.

5. An apparatus for detecting cloudiness of media having transparency, comprising at least one light source, a first target adapted to be illuminated by said light source, a second target, both said targets having transparent and dark portions, a container for the medium to be tested having at least tWo transparent walls, optical means including at least one objective for projecting an image of said first target across the medium to be tested onto said second target, means to periodically vary the incidence of illuminated and dark portions of the image formed by said first target with light and dark portions of said second target and photoelectric means adapted to be influenced by light passing said second target and to detect the relative modulation of said light passing said second target.

6. An apparatus for detecting cloudiness of media having transparency, comprising at least one light source, a first target adaptedto be illuminated by said light source, a second target, both said targets having transparent and dark portions, a first objective and a second objective disposed so as to contain the medium to be tested between them, said first target being arranged in the focal plane of said first objective and said second target in the focal plane of said second objective, means to periodically vary the incidence of illuminated and dark portions of the image formed by said first target with light and dark portions of said second target and photoelectric means adapted to be influenced by light passing said second target and to detect the relative modulation of said light passing said second target.

7. An apparatus as claimed in claim 4, having means for moving the image of the first target relative to the second target.

8. An apparatus as claimed in claim 4, wherein the two targets have parallel transparent and opaque stripes and means are provided for vibrat- 14 ing at least one of said targets in its own plane in a direction perpendicular to said stripes.

9. An apparatus as claimed in claim 4, wherein the two targets have central-symmetrical transparent and opaque stripes and at least one of said targets is rotatably mounted for being rotated by a motor around its axis of symmetry.

10. An apparatus as claimed in claim 4, wherein the two targets are structurally united into a single target at one side of the medium to be tested and reflecting means are provided at the other side of said medium.

11. An apparatus as claimed in claim 10, wherein said single target has transparent and dark portions in the form of parallel stripes and said reflecting means comprise two reflecting surfaces at a right angle to each other, the line of intersection of said two reflecting surfaces extending parallelly to said parallel stripes. 12. An apparatus as claimed in claim 4, wherein said transparent and dark] portions of one or both targets are formed by the inclined surfaces of at least one prism.

13. An apparatus as claimed in claim 4, wherein said transparent and dark portions of one or both targets are formed by the inclined surfaces of a plurality of parallel furrows of a transparent body.

14. An apparatus as claimed in claim 4, wherein the width of the transparent and dark portions of at least one target in at least one direction is small compared with the distance of said target from said objective.

15. An apparatus as claimed in claim 4, wherein the width of the transparent and dark portions of at least one target in at least one direction is smaller than A of the distance of said target from said objective.

16. An apparatus for detecting cloudiness of media having transparency, comprising two lamps, a source of alternating current, two rectifiers, said two lamps being connected to said source of alternating current across said two rectifiers, said two rectifiers being arranged so as to supply said two lamps with current of opposite polarity, and photoelectric means including at least one phototube arranged in the path of rays emanating from one of said lamps and passing through said medium to be tested in unbroken straight lines, and means adapted to detect the relative modulation of the light falling on said phototube.

1'7. A method to detect cloudiness of transparent media comprising the steps of causing light from at least one light source to pass across the medium to be tested onto at least one photoelectric cathode, periodically interrupting those rays of said light which pass through said medium in an unbroken straight line for each passage from one end to the other end of said medium, and detecting on said photoelectric cathode the ratio between variations of illumination occurring at the rate at which said rays are periodically interrupted and the average illumination.

18. An apparatus for detecting cloudiness of transparent media, comprising at least one light source, at least one photoelectric cell, optical means to direct light from at least one of said light sources to at least one of said photoelectric cells across the medium to be tested, means for periodically interrupting those rays of said light that pass said medium in straight lines, unbroken in the inside of said medium, and an amplifier sensitive to the ratio between the variation of i1- lumination on at least one of said photoelectric cells occurring at the rate at which said interslanti :mean wne iodic l i t r up esaid y and theayerage illumination on that cell.

- 1S An -apparatus as claimed "in --claim 18- in iphotoelectriclacathode and light from the other light sourceso as to by -passi said photoelectric cathode, means to cause the'light emission ofone otsaid two light sources to periodically-increase and decrease and means to cause the light emission of the other light source to increase when thelight emission of the first named light source decreasesand to decrease when the light emission from said first named light source increases,

an angamplifi res sitive' -t the atio between the variation ofillumination onsaid-phobofllectric-cathode occurring at-the rate atrwhich said light emission periodically increases and decreases and the averageillumination onsaid photoelectric cathode. V

JOSEPH C.

REFERENCES CITED The following references :are of;;recor d; in the file of this patent:

UNITED STATES "PATENTS Number Name Date "2,254,548 RuhIe-et al. g Sept. 2,1951 2,268,133 vCarlson v Dec. 32,1941 2,287,322 Nelson a, ,,June 2 3, 1952 2,339,053 Coleman Jan. 11', 19% 2,402,926 I-Ierschman June 25,1946

" :FOREIGN PATENTS Number Country Data =Germany May 15, 1941 

